The present invention relates to a master cylinder used in a hydraulic brake system for vehicles such as automobiles.
Conventionally, such a master cylinder includes a cylinder body having a cylinder hole, a piston slidably inserted into the cylinder hole, the piston partitioning a hydraulic chamber, a communicating passage provided in the cylinder body, the communicating passage communicating with a reserver, a relief port formed in the piston, the relief port performing communication between the communicating passage and the hydraulic chamber, and a seal member housed in a recess portion in an inner peripheral surface of the cylinder hole of the cylinder body, the piston slidably passing through the seal member, the seal member performing sealing between the inner peripheral surface of the cylinder hole and an outer peripheral surface of the piston, in which the communicating passage communicates with the relief port during non-actuation and the piston moves to block the communicating passage from the relief port using the seal member during actuation.
The relief port has a function as a throttle that suppresses a flow of a hydraulic fluid from the hydraulic chamber to the reserver before the seal member passes and the function ensures the brake responsivity at the early state of depression of the brake pedal. However, when a rapid liquid flow of the hydraulic fluid from the reserver side to the hydraulic chamber side is generated by a pressure reduction trend in the hydraulic chamber caused during the releasing of a sudden brake operation, the above throttle effect of the relief port prevents the fluid from being speedily supplied to the hydraulic chamber, possibly causing a return delay of the piston or a release delay of the brake. In addition, as recent brake control for vehicles, there is an automatic brake system that detects the state of a running vehicle in addition to braking operations by the driver and, based on the detection result, automatically applies a predetermined braking force to the wheels. In this case, the hydraulic pump sucks and pressurizes the hydraulic fluid via the communicating passage and the hydraulic chamber and supplies the hydraulic fluid to the brake device side of the vehicle. However, the above throttle effect of the relief port may make the sucking of the hydraulic fluid insufficient.
To solve this problem, there is a brake system having flow passage control means that has the first state in which a restricted flow passage using a small passage having a relatively smaller flow area than the relief port is formed in a communicating passage and the second state in which a bypass flow passage bypassing the small passage is formed in the communicating passage and performs switching between the first state and the second state by deforming or moving in the communicating passage when receiving a flow of the hydraulic fluid (see JP-A-2001-180471).